2008/8/26 Philip Pemberton : > Hi, > Well, I'm two days into my summer holiday, and I've already run out of things > to do on my current projects. So I figured I'd resurrect an earlier project > that I put on the backburner ages ago. > > A good few years ago, I bought a pair of Epson M-180 series impact printer > mechanisms. These are basically 24-character dot-matrix printers which use > five horizontal solenoids instead of the more typical single 9-pin vertical > head. The idea being that it speeds up printing, and allows graphics printing > where necessary. It's actually pretty speedy... for a 1980s vintage piece of > kit. A lot more power hungry than my Seiko MTP-series thermal mechs, and a bit > of a pig to design hardware around. Typical Epson kit really, fussy and finicky :) > > From my project notebook, I was having issues with the print head solenoids > holding down for too long: > > > Print quality still terrible. P/Head solenoids seem to be holding too long, > > 1N4001 diode snubber slowing decay. Removed diode - some improvement, but > > ULN2003 driver failed in short order, probably due to back-emf. Needs > > further work. > > I've been digging through Horowitz & Hill, and scanning Google for about 40 > minutes now, and haven't found any decent material on back-EMF suppression for > transistors used for driving inductive components. I've found tons of > references that boil down to "just use a 1N4001", a few that actually admit > that a '4001 will slow a relay's switch-off response "significantly", but > nothing on what to do to speed things up. > > Does anyone know of any good articles on this? I'd rather like to get the > M180s doing something useful - maybe hook one up to a PIC and use it to grab > screendumps and trace data from some of my testgear. > > Of course the easy answer is "give up and dig out a thermal printer module", > but they're text only (7-pin head, not the 8-pin graphics head) and frankly > it's far easier to get paper and ribbons for the Epson modules than it is to > get 4" wide thermal paper for the MTPs... > > Thanks, > -- > Phil. > piclist@philpem.me.uk > http://www.philpem.me.uk/ > -- Phil The problem is that when turned on, the solenoid stores energy. When you disconnect the current supply, this energy has to dissipate somewhere and the simplest method is to let the current loop back through the core and the energy will dissipate itself in the resistance losses of the windings. This is what the diode does but the resistance is usually low, so the current takes a while to decay. In order to get the current to die quickly, you have to add additional loss to the system. This could be done by adding an external resistance in parallel with the coil, but at the expense of increasing the on current. Or you can add series resistance in series with the diode, but at the expense of having to withstand a higher voltage. Note that when you switch the coil current, the current will attempt to flow at the same rate, regardless of the impedance it is facing. Useful solutions are;- Resistor or zener in series with flyback diode. No flyback diode but zener or mov or resistance (high enough t ensure dropout) to ground. MOV fitted in parallel with coil. Capacitor resistor to ground or in parallel with coil. (Also can be used to reduce EMC problems) Active circuit to switch coil current to energy store or dissipator or back to power supply. Combination of all of above. Which works best for you will depend on your circumstances and limitations. E.g. the size of the solenoid, the voltage rating of the switch, safety/regulatory requirements, EMC etc. One method that often works is to add a little resistance in series with the switching device gate/base (value depends on device type ~100 ohms might be a starting point for medium sized MOSFETS) Then connect a diode and zener series combination between the gate and the drain (base and collector ?). The zener should be selected to have a voltage a little less than the max rating for the switching element (plus gate turnon voltage). Then, when the device is switched off, the voltage pulse at the drain rises until the zener conducts, this then starts turning the switching transistor on and holds the drain at a voltage just below its rating while the energy dissipates. So the switching device dissipates the energy stored. If you are switching often, this may cause problems in temperature rise, but for infrequent switching works well. In your case you will need to heatsink the driver transistor but even so the above may be too much for it. It would be nice to be able to recover the energy from the solenoid, but I can't think of an easy way to do it. Maybe a current transformer feeding a diode bridge feeding back into the power supply. But that might limit turn-on speed also so wouldn't work. Maybe not a diode bridge but a single diode feeding back to the power supply and an anti-polarity diode to short the current transformer on turn-on? Or a FET switch operated off the printhead drive signal? Hope this helps. Richard P -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist